Horticulture/Ailanthus altissima

Ailanthus altissima (Tree of Heaven or Ailanthus) is a member of the quassia family, ../Simaroubaceae/, native to northeast and central China, and Taiwan, but now presently an invasive weed throughout much of the world. It is a deciduous tree which grows rapidly and can reach up to 25 m tall, rarely 35 m, with a trunk up to 1 m diameter, rarely 1.5 m diameter. The common name is a direct translation of the Indonesian name ailanto for a closely related species in the same genus. Other common names include china sumac, copal tree, stinktree and ghetto palm.

The tree is occasionally planted in heavily polluted areas as it is tolerant of both particulate and chemical pollutants, as well as saline soils. Outside those areas, it should never be planted due to its invasive nature.

Description
The bark of the tree is smooth and light grey, while the stems are reddish or chestnut. Its large, compound leaves are arranged alternately on the stem, and can be 30–60 cm long (occasionally up to 1 m long on vigorous young sprouts) and contain 11-33 leaflets, occasionally up to 41 leaflets. Each leaflet has one to three teeth on each side, close to the base. This helps distinguish it from sumacs (Rhus spp.).

The flowers are small, yellow-green to reddish, produced in late spring to mid summer in panicles up to 30 cm long. It is dioecious, with trees being either all male or all female. The seed is 5 mm diameter, encapsulated in a samara 4 cm long and 1 cm broad; the samara is twisted, making it spin as it falls, assisting wind dispersal. Female trees can produce more than 300,000 seeds in a year. All parts of the tree produce an unpleasant odour, suggestive of rancid cashews, with male flowers having the strongest smell.

In overall appearance, it is somewhat similar to some species of sumac; Staghorn Sumac ../Rhus typhina/ can be distinguished by sumac's red and slightly hairy stems, as well as leaves that are serrated, instead of having the base teeth of A. altissima. Smooth Sumac ../Rhus glabra/, also lacks the base teeth of A. altissima. Ailanthus also grows much taller than all sumac species. Seedlings of the Black walnut (../Juglans nigra/) can also sometimes be mistaken for this plant, but their trunks are more slender as a sapling.

Ecology
Ailanthus is an opportunistic species, thriving in full sun and in disturbed areas. It does, however, exhibit some shade tolerance. It spreads aggressively both by seeds and vegetatively, through root sprouts. It can resprout rapidly after being cut. Ailanthus is among the most tolerant of all tree species to pollution, including sulfur dioxide, and high soil acidity such as that from acid mine drainage (as low as pH 4.1). It has been noted as drought-tolerant, storing water in its root system. It is frequently found along highways and railroad tracks, in abandoned lots in cities, on abandoned mining sites, and in other areas where few trees can survive. Along highways it often forms dense thickets in which few other tree species are present.

Ailanthus produces allelopathic chemicals, which inhibit the growth of other plants. Resistance in various plant species has been shown to increase with exposure; populations without prior exposure to the chemicals are most susceptible to them. A few plants are resistant to these chemicals and form associations with Ailanthus in areas where it is dominant, such as along highways.

People have suggested many factors to Ailanthus' success at naturalising, including the absence of insects eating the plant, lack of exposure of native plants to the Ailanthus' allelopathy, and the widespread human disturbances which favour the tree. Regional variation has begun to show throughout its range, with trees in the colder northern regions producing heavier seeds than those in warmer regions.

Uses
Tree of Heaven is a popular ornamental tree in China, valued for its tolerance of difficult growing conditions, and its uses in Chinese traditional medicine. The bark is used to treat dysentery and other bowel ailments. A tincture of the root-bark has been used successfully in cardiac palpitation, asthma and epilepsy. The leaves are also used to feed silkworms of the moth Samia cynthia, which produces silk that is stronger and cheaper than mulberry silk, although with inferior gloss and texture. There are also records of the wood from this tree being used in China. Under the synonymous name "A. glandulosa", an extract of the bark is sometimes touted as an herbal homeopathic remedy for various ailments. However, taken in large doses, the bark extract is highly toxic.

It was first introduced to Europe (France and England) by a French Jesuit priest returning from Nanking in 1751. It was brought to the United States by William Hamilton, a gardener in Philadelphia in 1784 and soon became a favoured ornamental tree in parks and gardens. By 1840, it was available in many nurseries and was planted as an ornamental. The tree arrived in the Western U.S. with the Chinese immigrants who worked the gold mines in California. To this day many abandoned mines have large colonies of it.

In the landscape these trees can cause problems because of their aggressive nature and weak branching habits. As a rule, they should not be planted outside of their native range, and female trees should be killed or kept cut to the ground to prevent further seed production. Male trees can be grown as a fast-replenishing postwood crop or hedge by coppicing at the ground every few years.

Control
Tree-of-heaven has become a problematic invasive species in many areas with warm temperate climates, especially in North America, due to its aggressive spread, vigorous growth and allelopathic chemicals. It often grows directly up against a building or structure, where the roots can damage sewers and housing foundations. The trees grow rapidly and produce many offspring in their root vicinity. They also shed many small branches at regular intervals. It is an agricultural pest as well. These undesirable qualities often lead land and business owners to eradicate the plant.

Many different methods of control have been attempted and the best involve prevention as well as eradication. Means of eradication can be physical, thermal, managerial, biological, or chemical. A combination of these can be most effective, though they must be compatible. Physical methods are desirable due to their high selectivity, but are very labour intensive and thus more expensive. Hand-pulling is a highly effective way to remove young seedlings before the development of a tap root, but thereafter it is ineffective. Cutting and hand digging are options for larger trees, but the former will produce stump sprout which will need to be controlled later and the latter is very time consuming and is only practical for small infestations. Girdling, the removal of the cambial tissue with a hand axe or machete, is effective for very large trees, though re-sprouting often occurs. Thermal control, i.e. controlled burning, is also effective at removing the visible portion of trees, but sprouts will occur shortly after. These methods can be counter-productive if not performed regularly. On their own they are most effective in places with small infestations or in areas with fairly strong shade or competition. The root systems will eventually become exhausted and die if mechanical or thermal control is done thoroughly and consistently, though this may take several years.

Managerial control is not effective with controlling ailanthus as native trees cannot compete easily with the tree and it is unpalatable to animals that could potentially graze on it. Biological control, the use of insects or diseases, is also not in use for ailanthus eradication. Chemical control, however, is quite successful, especially when combined with mechanical methods. Foliar herbicide sprays are very effective when plants are in full leaf, but are difficult to use when desirable plants are in the vicinity. Also very large trees will be out of reach. Herbicides for this use include the non-selective glyphosate, though care must be taken as it is mildly toxic to animals and especially to aquatic life. Its trade names include Accord and Roundup, though the latter contains other harmful surfactants which may be more toxic than the herbicide itself. Triclopyr is another option and is selective for woody plants, making it a better choice for sensitive areas. It is also non-toxic to fish, though it can be toxic to waterfowl. It is sold under brand names such as Garlon. Foliar applications of glyphosate have been shown to be slightly more effective than triclopyr. Dicamba, imazapyr and metsulfuron methyl are also effective, but have not been tested extensively with ailanthus.

Other chemical methods for controlling ailanthus include a basal bark application of oil-soluble triclopyr in late winter or early spring. This method requires no cutting, but is only effective on trees with a diameter of 15 cm (6 inches) or less. For larger trees, an effective method is to cut off strips of bark at the trunk during the summer and spray a 100% concentration of triclopyr, only about 1 or 2 ml per cut, within a few minutes. The tree should not be cut all the way around in a ring, but rather only a ring with 3 to 6 cm (1 to 2 inch) pieces of living bark in between each cut. Cutting a full ring will kill the upper part of the tree and cause root suckers to sprout. Lastly, trees can be cut down and the stump treated with any of the above herbicides, though they must be applied immediately after cutting. This is most effective during the growing season.